1. Field of the Invention
The invention relates to a method for controlling the winding density of film rolls.
The winding density of a film roll is of central importance in the process of producing films. When winding or taking-up a film onto a reel or mandrel, a specific quantity of air is necessarily required between the individual layers. This air permits film shrinkage during storage of the film roll before processing and compensates for unevennesses in the profile. If a film web which inherently meets the specification is wound incorrectly, the result can be a total loss of the film roll owing to damage to the film web, in particular owing to storage. It is in this context that substantial quantities of air are also entrained with the film during winding of the film web. A portion of this entrained air escapes from the film roll during storage of the latter, and this can produce various defects in and on the film roll, such as sagging, stretching and transverse corrugations, which could cause the film to be ruined.
2. Description of the Related Art
In the device described in DE-C 32 65 570 (U.S. Pat. No. 4,576,344), a so-called pressure roller is used to press the film web running onto the reel against the film reel, the result being that air is entrained in smaller quantities than without this measure. The pressure exerted by the pressure roller can be controlled. With increasing winding speed, however, the air-displacing effect of the pressure roller decreases, and so it is necessary to compromise here between the winding speed and the entrainment of air.
European Patent 0 393 519 discloses a device for taking-up a film web onto a mandrel, including a pressure roller for feeding the film web to the film reel. In this case, the pressure roller and the film reel run at the same circumferential speed in opposite senses relative to one another. Two additional rollers are arranged axially parallel relative to the mandrel and to the pressure roller, and press the film web against the film roll and the pressure roller, respectively. In this case, the pressure roller has a smooth hard surface layer which has a mean roughness value R.sub.a &lt;0.4 .mu.m and a Brinell hardness&gt;10 HP 2.5/62.5. The roller bearing against the pressure roller is fixed during the winding operation and is pressed against the contact roller. It is connected to a cylinder at each end via an angle lever, and both cylinders are fastened on a pressure roller holder. The roller bearing against the film reel is movable mounted and guided by a cylinder which is pivotally mounted on the pressure roller holder. A spacer is mounted rotatably between the axes of the rollers and connects the two rollers to one another and keeps them at a distance from one another.
If the entrained quantity of air in a film roll is too low, shrinkage processes in conjunction with profile defects lead to stretching and thus damaging or ruining the film.
If the entrained quantity of air is too high, a multiplicity of other problems arise, such as displacement (i.e., lateral offset of the individual film layers) or central breakthroughs. Moreover, film rolls cannot be processed having an excessively high entrained quantity of air at high speeds, which generally leads to bottlenecks in processing capacity.
The winding density is to be understood as the ratio of the density of the taken-up plastic plus entrained air to the density of the pure plastic.